The rearrangement of epoxides is well known in the art. It is further known that acid-catalyzed rearrangement results in the formation of carbonyl products, whereas base-catalyzed rearrangement gives rise to allylic alcohols as products. See J. Org. Chem. 35, (1970) 1598.
When an acid catalyst is employed, two product-forming pathways are possible as shown in Scheme i. In Path a the internal epoxide bond (i.e., Bond #1) is cleaved and the hydrogen atom migrates from the terminal carbon atom to the internal carbon atom resulting in the formation of aldehyde 1. In Path b the external epoxide bond (i.e., Bond #2) is cleaved and the R.sub.2 group migrates from the internal carbon atom to the terminal carbon atom resulting in the formation of ketone 2. If R.sub.2 is a hydrogen atom, then the product is a methyl ketone possessing the same carbon skeleton as the reacting epoxide. On the other hand, if the R.sub.2 group is a carbon-bearing substituent, then the resultant ketone will possess a rearranged carbon skeleton. ##STR1##
The nature of the R.sub.2 group (i.e., its steric bulk, inductive or repulsive electronic effects, etc.), inherent characteristics of the particular epoxide (i.e., ring strain, dipole-dipole interactions, etc.) and the various properties of the acid catalyst will all have an effect on the extent each pathway is followed and the amount of aldehyde and ketone in the final product.
In the case of a 1,2-epoxide where the R.sub.2 group is a hydrogen atom and the various steric and electronic effects are minimal, it is difficult to find conditions which cause the rearrangement to occur exclusively by one of the two possible pathways. Known methods using catalysts such as mineral acids or Lewis acids usually give mixtures of aldehydes and ketones.
U.S. Pat. No. 3,855,303 discloses the rearrangement of terminal epoxides to the corresponding aldehydes containing a small amount of the corresponding methyl ketone. The process utilizes alkali metal perchlorates in combination with trialkyl- and triarylphosphine oxides as catalysts for effecting the rearrangement. The commercial feasibility of this process is questionable since no yields are disclosed and since a reaction time of 72 hours is required in the single illustrative example offered.
Organoleptically pure n-aldehydes, such as n-hexanal, n-heptanal, n-decanal, n-dodecanal, etc., are important and very useful ingredients in flavors and fragrances [see S. Arctander, "Perfume and Flavor Chemicals", Vols. I and II, Steffen Arctander, Publisher, Montclair, N.J. (1969)]. None of the processes described in the prior art provides a method of converting terminal epoxides to aldehydes without at the same time producing substantial amounts of ketone or requiring conditions that are not commercially feasible.
Since it is known in the art that terminal epoxides can be produced economically from terminal olefins, most of which are inexpensive and readily available from the petroleum industry, there is a need for an economical and commercially feasible process for converting such epoxides to aldehydes without, at the same time, producing a subatantial amount of the undesired ketone.